KVM: arm64: pvtime: steal-time is only supported when configured

Don't confuse the guest by saying steal-time is supported when
it hasn't been configured by userspace and won't work.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200804170604.42662-2-drjones@redhat.com